Currently, use of diesel engines increases day by day, making the pollutant emission from the diesel engines to cause various environmental problems. In exhaust gas from the diesel engines, not only nitrogen oxides NOx and sulfur oxides SOx, but also fine particles (for convenience's sake, will be called as “soot particles”), cause problems. Therefore, in order to prevent the soot particles from emitting to the air, a soot particle collecting filter is mounted to an exhaust gas flow passage for collecting the soot particles which are solid particles to be emitted to an outside of the combustion apparatus, and regenerated, periodically or continuously. At the time of regeneration of the filter, the soot particles collected at the filter is burned and discharged as a gaseous substance, for making the filter to return to an initial state.
Presently, as the soot particle reducing systems, there are two systems in use, having shapes of soot collection filters and collection characteristics different from each other. That is, one is a system in which the soot particles are filtered by a filter having a large surface with holes smaller than a size of the soot particle (hereafter called as a “Surface filtration system”) and the other one is a system in which a filter having a stack of a predetermined volume of grains, such as a plurality of metal or ceramic beads, collects the soot particles throughout entire volume (a “Volumetric filtration system”). In the volumetric filtration system, the soot particles deposit on a boundary surface between the grains and pores in a course the exhaust gas containing the soot particles pass through grain layers.
Referring to FIGS. 1 and 2, the surface filtration system uses a filter having a fixed shape and structure, such as a honeycomb shaped ceramic carrier filter, a metal mesh filter, and a filter fabricated by weaving or sintering high temperature fiber or ceramic fiber (hereafter called as a “structured filter”). Though the surface filtration system has an excellent soot collection performance, the surface filtration system has weak mechanical and thermal durability due to properties of the ceramic and metal of the filter. Moreover, since the surface filtration system requires a structure having a large surface area for reducing an initial back pressure, most of the surface filtration systems have a honeycomb shape, and are expensive. And, if emission of the soot particles is great and if the soot particles collected is great, since holes through which the exhaust gas flows become the smaller gradually, the structured filter causes sharp increase of the back pressure, impeding smooth operation of the engine.
FIGS. 1 and 2 illustrate a filter of cordierite, and a ceramic filter of silicon carbide, which are used the most as the structured filter up to now, respectively. The filters have problems in high temperature durability, and high temperature thermal expansion. The cordierite ceramic structured filter has problems in a low allowable temperature gradient and durability coming from melting at a high temperature. In order to overcome those problems, a ceramic filter of the silicon carbide which can endure at a high temperature is used mostly. However, the silicon carbide ceramic filter has a high thermal expansion coefficient, and has a problem of a temperature gradient at a high temperature. Therefore, a whole filter 2 can not be fabricated as one unit, but small filter segments 20 are combined to complete the whole filter 2. Thus, the silicon carbide ceramic filter has a deep-seated trouble in which the filter cracks and damaged easily due to a bonding material (See FIG. 3). In order to improve the problems of the surface filtration system, U.S. Pat. Nos. 3,937,015, 4,912,076, 4,759,918, 5,497,620, 4,535,589, and 4,852,349 are suggested. Basically, above patents suggest the surface filtration system.
In order to solve the problems of the surface filtration system described above, the applicant developed devices and methods for reducing the soot particles, of the volumetric filtration system, and filed a plurality of patents (Korea Patent Application Nos. 10-2000-3814, 10-2001-39523, 10-2002-1902, 10-2005-92078, and so on). As can be known from FIG. 4, in the volumetric filtration system, a plurality of grains 210, such as metal/ceramic beads, are stacked to make the filter (hereafter “grain layer filter” for convenience's sake) to have a predetermined volume, for collecting the soot particles with an entire volume. That is, the grain layer filter embodies a volumetric filtering mechanism as the soot particles deposit at a boundary surface between the grains and pores of the filter in a course the gas carrying the soot particles passes through the grain layer. The grain layer filter has no stress caused by a thermal gradient in the filter, an excellent durability with respect to a high temperature environment and mechanical vibration, a very low back pressure increasing rate, and a very excellent economic aspect. However, in comparison to the structured filter having the same or almost same volume, the grain layer filter has a drawback in that an initial back pressure is very high resulting in a large filter volume for overcoming the problem of the initial back pressure. Moreover, in order to obtain a high level of soot particle collection efficiency in the volumetric filtration system, the grain layer filter requires having a large volume. In conclusion, development of device and method for reducing soot particles are required for solving the drawbacks both of the structured filter and the grain layer filter while maintaining advantages thereof.